Device containing plurality of smaller MEMS devices in place of a larger MEMS device

ABSTRACT

Embodiments disclosed herein generally include using a large number of small MEMS devices to replace the function of an individual larger MEMS device or digital variable capacitor. The large number of smaller MEMS devices perform the same function as the larger device, but because of the smaller size, they can be encapsulated in a cavity using complementary metal oxide semiconductor (CMOS) compatible processes. Signal averaging over a large number of the smaller devices allows the accuracy of the array of smaller devices to be equivalent to the larger device. The process is exemplified by considering the use of a MEMS based accelerometer switch array with an integrated analog to digital conversion of the inertial response. The process is also exemplified by considering the use of a MEMS based device structure where the MEMS devices operate in parallel as a digital variable capacitor.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims benefit of U.S. Provisional Patent ApplicationSer. No. 61/112,521, filed Nov. 7, 2008, which is herein incorporated byreference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

Embodiments of the present invention generally relate to a device and amethod of using the device where a plurality of smallermicro-electromechanical system (MEMS) devices replace a single, largerMEMS device.

2. Description of the Related Art

MEMS devices are routinely produced using semiconductor processing. Thisallows accelerometers, pressure sensors, low resistance current switchesor RF switches, variable capacitance devices, resonators and otherdevices to be manufactured cheaply. For many of these devices to workwith the desired physical properties they usually cover an area ofseveral hundred microns square. It is then very difficult to seal such adevice in a cavity using the typical interlayer dielectrics found in theback end or the metallization found in the back end because these are ofthe order of 1 micron thick. In many radio frequency applications, it isdesirable to have a variable capacitor that can be used, for example,for tuning a resonant LRC circuit in an antenna module. The variablecapacitor can be used for switching between carrier frequencies in amobile phone or other apparatus that may be used at a number ofdifferent frequencies.

A low pressure cavity is usually required for the operation of thesedevices, which leads to a pressure on the roof of the cavity. With theMEMS devices being over 100 microns wide, the cavity will collapse underthe external pressure. To solve this problem MEMS devices are separatelypackaged which can double the price of a device. In some applications itwould be advantageous to have a MEMS device on the same chip as a microcontroller or other logic device, but that is not possible because itmust be separately packaged.

MEMS based accelerometers have been built using a variety ofmicro-machining techniques for many years. Most of these MEMS basedaccelerometers rely upon precise micro-machining of a single proof masssuspended by beams to set the sensitivity and signal range of thesensor. The typical sensing scheme is capacitance based, but othersensing strategies have been used. The size of the proof mass, isnormally of the order of 100 microns in at least two dimensions, and maybe a few microns in size in the third spatial dimension. Once thesuspension system is added, careful packaging strategies ranging fromreverse wafer bonding to full hermetic packages are required. Thesecomplex packaging strategies add to the cost of the sensor and limit thesensors ability to be fully integrated in a standard back and of theline (BEOL) or standard packaging flow.

Therefore, there is a need for a device integrated into the chip and amethod for its manufacture.

SUMMARY OF THE INVENTION

Embodiments disclosed herein generally include using a large number ofsmall MEMS devices to replace the function of an individual larger MEMSdevice. The large number of smaller MEMS devices perform the samefunction as the larger device, but because of the smaller size, they canbe encapsulated in a cavity using complementary metal oxidesemiconductor (CMOS) compatible processes. Signal averaging over a largenumber of the smaller devices allows the accuracy of the array ofsmaller devices to be equivalent to the larger device. The process isfirst exemplified by considering the use of a MEMS based accelerometerswitch array with an integrated analog to digital conversion of theinertial response. In the second example we will discuss breaking up alarge MEMS based variable capacitor, in which the capacitance iscontrolled by the gap between a MEMS switch and a landing electrode,into an array of smaller MEMS variable capacitors which are connected inparallel and are either stuck up away from the landing electrode, orstuck down to a thin oxide over the landing electrode. Thus the arrayacts as a digital variable capacitor.

In one embodiment, a MEMS device may be used as a variable capacitorsuch that the capacitor is broken up into an array of small MEMScapacitors. These smaller capacitors may be housed in a cavity that canbe fabricated in a CMOS compatible process in the back end metallizationof a semiconductor integrated device. Each smaller capacitor has twowell defined capacitance states: either fully pulled into a landingelectrode with a thin insulator on top or fully free standing with thesignal line away from the cantilever. By breaking the capacitor up intoa large number of smaller capacitors that act in parallel, a desiredcapacitance can be created as long as the individual capacitance of eachvariable capacitor is small enough to give the resolution in capacitancethat is required.

In another embodiment, a device structure is disclosed. The devicestructure may include a substrate and a plurality of layers formed overthe substrate. A first layer of the plurality of layers may bound one ormore cavities formed within the structure between the substrate and theplurality of layers. The structure may also include a plurality ofdevices disposed over the substrate and within the one or more cavities.

In another embodiment, a method of using a device structure isdisclosed. The device structure may include one or more cavities and aplurality of devices within the one or more cavities. Each device mayinclude a corresponding landing electrode. The method may includeapplying a first electrical bias to the plurality of landing electrodesto move the plurality of devices from a first position spaced from aplurality of landing electrodes into a second position in contact withthe plurality of landing electrodes. The method may also include movingone or more of the plurality of devices and detecting a number ofdevices that remain in contact with the plurality of landing electrodesfollowing the acceleration. The method may also include applying asecond electrical bias to the plurality of landing electrodes to movethe plurality of devices into the second positions.

In another embodiment, a method of using a device structure isdisclosed. The device structure may include one or more cavities and aplurality of devices within the one or more cavities. Each device mayinclude a corresponding landing electrode disposed within the cavity.The method may include moving all devices to a first position spacedfrom the landing electrodes and moving one or more of the plurality ofdevices. The method may also include detecting a number of devices thathave moved to a second position in contact with the plurality of landingelectrodes following the acceleration and moving all devices to thefirst position.

In another embodiment, a method of operating a digital variablecapacitor is disclosed. The digital variable capacitor has a pluralityof micro electromechanical devices formed within a cavity. The methodincludes moving a plurality of cantilevers from a first position spaceda first distance from an RF electrode to a second position spaced asecond distance from the RF electrode. The first distance is greaterthan the second distance. The method also includes moving the pluralityof cantilevers to the first position. The method additionally includesmoving the plurality of cantilevers to a third spaced a third distancefrom the RF electrode, the third distance is greater than the firstdistance.

BRIEF DESCRIPTION OF THE DRAWINGS

So that the manner in which the above recited features of the presentinvention can be understood in detail, a more particular description ofthe invention, briefly summarized above, may be had by reference toembodiments, some of which are illustrated in the appended drawings. Itis to be noted, however, that the appended drawings illustrate onlytypical embodiments of this invention and are therefore not to beconsidered limiting of its scope, for the invention may admit to otherequally effective embodiments.

FIGS. 1A-1I show of an encapsulation method in accordance with anembodiment of the present invention.

FIG. 2A is a schematic cross sectional view of a structure prior tosealing the cavities.

FIG. 2B is a schematic cross sectional view of the structure 2B during asputter etch process.

FIG. 2C is the structure of FIG. 2A after sealing the cavities.

FIG. 3 is a diagram of an asymmetric switch according to one embodiment.

FIG. 4 is a diagram an accelerometer of an array housed in amicro-cavity.

FIG. 5 is a diagram of the expected distribution of variation for atorsion hinge.

FIG. 6 is a diagram showing the distribution of variation as a functionof contact stiction.

FIG. 7 shows the combined distribution of the pull off force due to thevariation of the adhesion force and the variation of the spring constantof the torsion arm.

FIG. 8A shows the distribution of Ft by design.

FIG. 8B shows the distribution of Fs-Ft including design variations.

FIG. 9 shows the measured acceleration at different time intervals forone embodiment.

FIG. 10 is a diagram of an asymmetric switch according to anotherembodiment.

FIG. 11 is a top view of a torsion cantilever with the two torsion sidearms and a large proof mass.

FIG. 12 is a schematic diagram of a circuit for an accelerometeraccording to one embodiment.

FIG. 13 is a schematic diagram of a small accelerometer from an arrayaccording to one embodiment.

FIG. 14 is a schematic top view of control electrodes and an RFelectrode according to one embodiment.

FIG. 15 is a schematic top view of a cantilever over control electrodesand an RF line according to one embodiment.

FIG. 16 is a schematic top view of multiple MEMS devices arranged alongan RF electrode according to one embodiment.

FIGS. 17A-17C are schematic cross sectional views of a MEMS capacitorswitch in the free standing state, the down state and the up stateaccording to one embodiment.

To facilitate understanding, identical reference numerals have beenused, where possible, to designate identical elements that are common tothe figures. It is contemplated that elements disclosed in oneembodiment may be beneficially utilized on other embodiments withoutspecific recitation.

DETAILED DESCRIPTION

Embodiments discussed herein generally include using a large number ofsmall MEMS devices to replace the function of an individual larger MEMSdevice or a digital variable capacitor. Solutions discussed hereininclude breaking up a single MEMS device or a variable capacitor into anarray of smaller MEMS devices that perform the same function as thesingle larger MEMS device variable capacitor. Each individual MEMSdevice in the array may only be a few microns across and so can behoused in a cavity that fits within the back end metallization. The roofof the cavity can be 1 micron thick interlayer dielectric for example,and because it is only a few microns wide, the atmospheric pressureoutside is not large enough to cause any significant deformation of thecavity roof, even for an evacuated cavity.

To illustrate why an array of small MEMS devices acting to measure somephysical phenomena has an advantage over a single large MEMS device,some of the embodiments discussed herein show the application of a MEMSaccelerometer, but the present invention of using a plurality of smallerMEMS devices to replace a larger MEMS devices is not limited to thisapplication.

The present invention could be used to make a fast, low voltage currentswitch consisting of many small cantilevers in parallel, or an RFswitch, or a variable capacitor could also be broken up into an array ofsmaller switches working in parallel. The advantage being that byreplacing the larger MEMS device with a large number of smaller devices,each one can be reduced to a size that allows them to be encapsulated onchip using a CMOS compatible process of deposition, lithography andetching. By scaling down all the devices the switching voltages can bereduced and the speed of switching can be increased.

The smaller MEMS devices can also be housed in their own cavity definedin the interlayer dielectric of a standard CMOS process. By usingtemperatures less than 400 degrees Celsius to fabricate the MEMS devicesand cavities, they can be easily integrated into the metallizationlayers of a standard CMOS process allowing MEMS devices to beincorporated on a CMOS chip. This reduces the cost of production becausea separate MEMS chip with more expensive MEMS packaging is no longerrequired.

By fabricating a micro-cavity around the individual MEMS devices in theCMOS fabrication facility it is possible to remove the sacrificial layeraround the MEMS device and seal it in the same processing tool, whichmeans the device environment is controlled at a low pressure andavoiding exposure to oxygen or water vapor for example. This allowstransition metals to be used in the cavity without oxidizing so thatthey have low resistance contact surfaces.

An array or ensemble of MEMS switches where each switch or multipleswitches are in their own internal cavity within the CMOS is disclosed.By using an array of smaller switches, an integration scheme can be usedwhere individual switches or small groups of switches can be embeddedinto small cavities in between the metal layers typically found insemiconductor processing.

Sensing can be achieved either through measuring the change ofcapacitance of the array of cantilevers as they move under accelerationforces, in the same way as the capacitance of a single larger device ismeasured or in a number of other ways. However the large number ofswitches offers a new measurement technique consisting of the simple actof checking for electrical continuity of each switch during a finitetime sample interval. Inertial loads such as acceleration cause apercentage of switches to change from closed to open during the sampleperiod. When a single switch changes state, this forms a digital signalevent. The raw percentage of the collection of switches which changestate forms an integer number that represents the magnitude of theanalog inertial input. The sensitivity of the collection of switchesdepends then upon the switch design, adhesion distribution of thecontact electrode, and the total number of switches. The sensitivity ofthe switches to an acceleration event can be increased by applying a DCpull off voltage which tries to overcome the adhesion of the other endof a rocker arm. Then a small extra force causes the cantilevers to moveto the point where they are pulled off the contact electrode. In thisway smaller acceleration values can be measured. The device can also beoperated in the other direction where the cantilever is free standingand an acceleration will cause it to switch on. A DC voltage can then beused on a close by electrode to apply a DC electrostatic force whichtries to pull the cantilever into contact then a small extraacceleration causes the cantilever to switch on.

With reference to FIGS. 1A-1I, a method of forming a device within amicro-cavity will now be described. In order to describe how to connectan element within the micro-cavity to a circuit outside themicro-cavity, a fixed end MEMS cantilever fabrication is shown. As willbe appreciated, any other suitable device could be encapsulated in themicro-cavity including accelerometers, variable capacitors or RFswitches and three way switches. As will also be appreciated, theembodiment described hereafter can be fabricated in any BEOL thatemploys chemical mechanical planarization (CMP) of inter-metaldielectric (IMD) films.

FIG. 1A shows the result of the first step of a method in accordancewith the present invention. The first step consists of using known CMOSprocess steps to fabricate a wafer up to a predetermined metal levelwithin the aluminum CMOS BEOL. The CMOS BEOL comprises an inter-metaldielectric layer 102 having metal channels 104 therein. In oneembodiment, the dielectric layer 102 may comprise silicon dioxide. Thevias 106 are fabricated using known methods such as conventionallithography and etch processes. The vias 106 may comprise a liner layerand a metal fill. In one embodiment, the liner may comprise titanium. Inanother embodiment, the liner may comprise titanium nitride. In oneembodiment, the fill material may comprise tungsten. In anotherembodiment, the fill material may comprise copper. In anotherembodiment, the fill material may comprise aluminum. The vias 106 may becapped with a titanium and/or titanium nitride layer. The titaniumand/or titanium nitride layer may be patterned using conventionallithography and etch processes to form a plurality of electrodes 108,one of which will be the lower electrode of the MEMS cantilever device.

FIG. 1B shows the result of the second step of the method. This stepcomprises coating the lower half of the first sacrificial layer 110 onthe wafer surface over the patterned electrodes 108. As this layer isthe first sacrificial layer 110, its thickness will be chosen to permitthe operation of the device within the resulting cavity. In oneembodiment, the first sacrificial layer 110 may have a thickness betweenabout 30 nm and about 500 nm. The first sacrificial layer 110 maycomprise a high temperature spin-on organic film. However, other filmssuch as silicon nitride, silicon dioxide, amorphous silicon andamorphous carbon, can be employed to the same effect. Other depositionmethods that may be employed include plasma enhanced chemical vapordeposition (PECVD), chemical vapor deposition (CVD), physical vapordeposition (PVD), and atomic layer deposition (ALD). A spin on firstsacrificial layer 110 may flow over any irregularities in the underlyinglayers, thereby producing a flat layer where the thickness of the filmdepends on the height of the underlying material.

FIG. 1C shows the result of the third step of the method. The third stepcomprises the patterning of a via structure 112 in the first sacrificiallayer 110 to form an interconnect from the underlying CMOS to the anchorpoint of the cantilever device. FIG. 10 shows the result of the fourthstep of the method, which comprises the deposition of a conductive layeron top of the first sacrificial layer 110. In one embodiment, theconductive layer may comprise titanium nitride. In other embodiments,the conductive layer may comprise a titanium aluminum compound, atitanium aluminum nitride compound, polysilicon, silicon, any conductivematerial, and combinations thereof. In another embodiment this layer maybe made of a combination of conducting and insulating parts. Theconductive layer will form the cantilever of the cantilever device. Oncedeposited, the conductive layer may be patterned using conventionallithography and etch processes in order to form the shape of thecantilever 114. In one embodiment, the conductive layer is patterned bydepositing a photoresist layer thereon, exposing the photoresist,developing the photoresist, removing the developed (or undevelopedphotoresist) to form a mask, and then exposing the conductive layerthrough the mask to an etchant to remove undesired portions of theconductive layer. Thereafter, the mask may be removed. In oneembodiment, a hard mask may be used.

FIG. 1E shows the result of the fifth step of the method, whichcomprises coating the wafer surface with the second sacrificial layer116 in order to cover the patterned cantilever 114. The depositing ofthis second sacrificial layer 116 effectively seals the MEMS elementprior to the creation of the micro-cavity. The second sacrificial layer116 may be deposited in a manner similar to the method of depositing thefirst sacrificial layer 110. Additionally, the second sacrificial layer116 may comprise one or more of a high temperature spin-on organic film,silicon nitride, silicon dioxide, amorphous silicon or amorphous carbon.In one embodiment, the second sacrificial layer 116 may comprise thesame material as the first sacrificial layer 110. In another embodiment,the second sacrificial layer 116 may comprise a material different thanthe first sacrificial layer 110.

FIG. 1F shows the result of the sixth step of the method, whichcomprises the patterning of the first sacrificial layer 110 and thesecond sacrificial layer 116 in order to form the shape of themicro-cavity 118. The shape and dimensions of the patterned micro-cavity118 depend on the structure which is to be fabricated. The firstsacrificial layer 110 and the second sacrificial layer 116 may bepatterned by conventional lithography and etching methods. For example,a photoresist layer may be deposited over the second sacrificial layer116. Then, the photoresist may be exposed and developed. Thereafter, thedeveloped (or undeveloped) photoresist may be removed to form a mask.Then the second sacrificial layer 116 may be exposed to an etchantthrough the mask to remove undesired portions of the second sacrificiallayer 116. The first sacrificial layer 110 may be etched in the samestep as the second sacrificial layer 116. For example, if the secondsacrificial layer 116 and the first sacrificial layer 110 comprise thesame material, the same etchant may be used. However, if differentmaterials are used, then different etchants may be needed to etch theseparate layers.

FIG. 1G shows the result of the seventh step of the method. In the step,the CMOS BEOL flow is continued and the next metal layer 120 isdeposited. The metal layer 120 may be blanket deposited over the CMOSBEOL and the patterned second sacrificial layer 116 followed by anetching process. In one embodiment, the metal layer 120 may be patterndeposited through a mask. The metal layer 120 may be in contact with oneor more electrodes 108 as well as the dielectric layer 102. Prior topatterning the metal layer 120, the metal layer 120 may encapsulate themicro-cavity 118. Once deposited, the metal layer 120 is then patternedand etched using conventional lithography and etch processes discussedabove. Following the patterning and etching of the metal layer 120, themetal layer 120 may be in contact with an electrode 108 outside of thecavity area 118 to provide the electrical connection to metallizationlayers to be formed above the micro-cavity 118. Additionally, the metallayer 120, after etching, may have one or more openings through thesidewalls that are filled with either the second sacrificial layer 116,the first sacrificial layer 110, or both. The opening in the sidewallpermits an etchant to reach the micro-cavity 118 and remove the firstand second sacrificial layers 110 and 116. In another embodiment,portions of either the second sacrificial layer 116, the firstsacrificial layer 110, or both may extend beyond the sidewalls of themetal layer 120 as release tabs.

The result of the eight step of the method is shown in FIG. 1H. Thisstep comprises the removal of the patterned sacrificial layer from thewafer surface in order to leave the freestanding MEMS device. In oneembodiment, the first sacrificial layer 110 and the second sacrificiallayer 116 may be removed using a dry plasma etch. In another embodiment,the first sacrificial layer 110 and the second sacrificial layer 116 maybe removed using the same etchant. In another embodiment, the firstsacrificial layer 110 and the second sacrificial layer 116 may beremoved using different etchants. The etchants used to remove the firstsacrificial layer 110 and the second sacrificial layer 116 may be thesame etchants used to pattern the first sacrificial layer 110 and thesecond sacrificial layer 116. The etchant or etchants are delivered tothe first sacrificial layer 110 and the second sacrificial layer 116through the sidewalls of the metal layer 120 or directly to releasetabs.

In one embodiment, the sacrificial layers 110, 116 may be removed byetching a hole through metal layer 120 to expose a portion of one ormore of the sacrificial layers 110, 116. The hole may be through themetal layer 120 such that a release hole is formed through the top ofthe micro-cavity 118. In another embodiment, the metal layer 120 may bepatterned such that the sacrificial layers 110, 116 are exposed from thetop, but off to the side of the micro-cavity 118. In another embodiment,the metal layer 120 may be patterned such that the sacrificial layers110, 116 are exposed from the side of the micro-cavity 118.

FIG. 1I shows the result of the ninth step of the method. This stepcomprises depositing an IMD or dielectric layer 122 in order toeffectively seal the fabricated micro-cavity 118 within the CMOS BEOL.The dielectric layer 122 may comprise silicon dioxide. The dielectriclayer 122 seals any openings into the micro-cavity 118 to encapsulatethe cantilever. As will be appreciated, the final shape of themicro-cavity 118 is dependent upon the structure which is to becontained within it. The height of the micro-cavity 118 is less than thetotal height of the dielectric layer 122 such that the micro-cavity 118fits within the dielectric layer 122. Therefore, the micro-cavity 118may be formed in a structure without altering the process flow for latermetallization layers. By fitting the micro-cavity 118 within thedielectric layer 122, no additional processing above the dielectriclayer 122 is necessary than would occur in absence of the micro-cavity118. In some embodiments the cavity may take up a height of more thanone interlayer dielectric. The subsequent metal layers can then run ontop undisturbed.

In FIG. 2A, both the substrate 202 and layer 204 will be sputter etched,while layer 206 and layer 208 may be sputter etched during the initialstages of the process until the surfaces are adequately coated by thesputtering and redeposition of layers 202 and 204. The resultingmaterial will be redeposited in order to seal the cavity 210, therebyforming a redeposition layer. In one embodiment, the layer 204 maycomprise a hard mask layer. The substrate 202 is locally sputter-etchedat the bottom of the release hole. Material from layer 204 may also besputter etched. The material from layer 204 may be redeposited onto thesubstrate 202 and along the sides of layers 206, 208 within the trench.The redeposited material from layer 204 may also sputter etch and helpseal the cavity 210. Thus, the material that may seal the cavity 210 maycome from the substrate 202, the layer 204, or even layers 206 and 208.In other words, the material that seals the cavity 210 comes frommaterial already present on the structure at the time the sealing isinitiated. A separate deposition such as CVD or even sputtering from asecondary source such as a sputtering target separate from the structureor gaseous precursors is not necessary. The substrate material may bechosen to suit the requirements of the redeposition layer. In oneembodiment, the substrate material may comprise an oxide. In otherembodiments, the substrate material may comprise silicon nitride, ametal, polysilicon, and combinations thereof.

As shown in FIG. 2B, the plasma is located away from the substrate 202.Thus, the gases used for sputter etching should not be acceleratedtoward the external target but toward the substrate 202. This can beperformed on an apparatus where the substrate 202 can be negativelybiased, for example when a RF bias is applied to it, with respect to theplasma.

Some of the sputtering gases are ionized in the plasma and acceleratedtoward the substrate 202. The materials being in the line-of-sight ofthese accelerated particles will be sputter etched (or sputtered) whenthe accelerated particles will reach the surface. They will then beexpelled in different directions. Some of the expelled particles will beemitted back into the plasma, others will be redeposited on the sidewalland on the passage entrance.

FIG. 2C shows a plurality of cavities 210 sealed with material 212 thathas been redeposited after sputter etching. The substrate 202 has beenlocally sputter-etched at the bottom of the via-like structure. In oneembodiment, the substrate 202 provides most of the material 214 beingredeposited toward the sidewall and the passage 216. The substratematerial can be chosen depending on the requirements for the redepositedlayer.

Layer 204 would also be sputter etch redeposited. Care should be takenin determining the material and thickness of the top layer of themultilayer stack as it would undergo most of the ion bombardments thatoccur during the sputter etching. Because the sputter etch rate is angledependant, some facets 214 may form at the corner of the layer 204.These facets 214 will move further away from one another as the sputteretch pursues. However, at a point of time, enough material would havebeen deposited at the passage 216 to close it. An etch stop layer havinga low sputtering rate can be used under layer 204 if desired. This wouldavoid etching layer 208 if required and limit the amount of facetingoccurring during sputter etching. It can also be used to tune the ratioof resputtered material coming from the top versus the resputteredmaterial coming from the bottom if necessary. Because the sputteretching occurs in an inert atmosphere, the cavity will be filled withinert gases rather than deposition gases.

An embodiment of the present invention to implement an accelerometerwould consist of a multitude of asymmetric rotating switches where eachswitch acts as the proof mass with torsion elements acting as thesuspension members. Making each switch asymmetric with respect to thetorsion legs allows for inertial loads (accelerations) to createreaction moments about the center of rotation (torsion leg axis). Asimple free body diagram of the asymmetric switch is shown in FIG. 3.

Referring to FIG. 3, this is a schematic diagram of an individualaccelerometer in the array. A proof mass on the left is free to rotateabout the torsion spring under the acceleration A. The centre of mass ofthe proof mass is a distance a away from the point of rotation. Thecontact bump is on the right and there is a restoring torsion momentwith a lever arm of length b from the torsion point and a torsion forceFt.

Referring to FIG. 4, this is a diagram of one of the accelerometers ofthe array housed in a micro-cavity 408 which has a low pressure of gasinside. Electrode 401 is a pull up electrode for the proof mass.Electrode 405 is a pull down electrode that allows the accelerometer tobe reset by pulling the contact 410 off the landing electrode 404. Thetouch down of the accelerometer is sensed by measuring the contactresistance between electrode 404 and contact 410. Electrical contact isalso made to the contact 410 and proof mass via the torsional spring arm403. The device is housed in a cavity which has a roof and walls 406fabricated on sacrificial layers which can be removed with a gas etchthrough release channels 411. These release channels are sealed withdeposition layer 409. The device sits on the interlayer dielectric 407of the underlying CMOS chip and the cavity is embedded in the nexthighest level of interlayer dielectric 412.

From FIG. 3, acceleration (A) of the proof mass (M) causes a momentabout the torsion spring axis that adds to the moment stored in thetorsion springs. The stiction force (Fs) creates an opposing moment. Asimple moment balance about the torsion axes yield the followingcondition for a switch to remain open or closed. The switch becomes openwhen:M*A*a+Ft*b>Fs*b

The switch remains closed when:M*A*a+Ft*b<Fs*b

The degree of asymmetry (ratios of a & b) and the size of the mass alongwith the design of the torsion legs (Ft) and contact adhesion (Fs)become design parameters to achieve the desired range of the sensor.

To sense small accelerations, it is preferred to adjust the torsion legsto produce a pull away force (Ft) at or near the expected value of thecontact adhesion (Fs). In this manner, approximately 50 percent of thetotal number of switches in the array will spontaneously release (act asa volatile switch) after the entire array is electrostatically forcedclosed. The remaining 50 percent of the switches would stay closed(i.e., act as a non-volatile switch). The 50 percent number is a resultof the normal distribution of values of (Ft) and (Fs) of the entirearray of switches.

The torsion legs, for example, would have a distribution for (Ft) asshown in FIG. 5 which shows the distribution of pull up forces at thecontact due to variations in the torsion hinge. This variation isproduced from small variations in etch dimensions of the legs,variations in the spacer height between the switch and the contactelectrode, and variations in the metal deposition thickness andcomposition used to create the legs. These variations combine togetherto create an overall variation of the stored strain energy in thetorsion leg which in turn creates a variation in the force the legsexert on the contact.

The contact adhesion would have a similar distribution as shown in FIG.6 which shows the distribution in the stiction force as a result ofvariations in the morphology of the contact. This contact adhesionvariation is a result of the size and shape of the contact area. Thematerials used in the contact as well as the surface roughness of thecontact. The voltage and/or amperage used to read the contact couldcontribute to this variation. Contact conditioning may be used asanother technique to manipulate the adhesion distribution.

Contact conditioning is a process whereby the surface roughness andcontact cleanliness can be altered by pulling the cantilever so that itmakes contact with the landing electrode and then increasing the pull involtage further which causes the surfaces to be pushed to more intimatecontact. This causes more asperities on one surface to adhere to theother resulting in a larger adhesion force. The distribution of thedifference between the switch mechanical characteristics and the contactsurface adhesion is also normal with a wider standard deviation.

FIG. 7 shows the combined distribution of the pull off force due to thevariation of the adhesion force and the variation of the spring constantof the torsion arm. The resolution of the sensor array is then the widthof the difference distribution divided by the total number of switchesin the array. For example, if an individual switch can resolve a shockof 10,000+/−1,000 Gs in the digital sense (i.e., closed to opencondition is met) then 10000 switches all centered around the sameconditional state can resolve G levels in the 10,000+/−0.1 G range.

The width and even shape of this distribution can also be controlled bychanging the design of each cantilever so that they are not designed tobe the same. With a whole range of different cantilever designs thedistribution can be made to be a wider normal distribution for exampleor a different shaped distribution as illustrated in FIGS. 8A and 8B.FIG. 8A shows the distribution of Ft by design. FIG. 8B shows thedistribution of Fs-Ft including design variations.

The trace in FIG. 8A shows an example of the distribution function forpull off forces that could be created by design changes over the arrayof cantilever. The curve in FIG. 8B shows the distribution in pull offforces taking the designed variation in torsion pull off force intoaccount as well as the random variation in the torsion force due to thenormal variations in the fabrication process and the random variationsin adhesion energies due to the variations in surface morphology of theregion of the cantilever that sticks and un-sticks during operation.

All the sources of variation described so far are time invariant andproduced during the construction of the array or ensemble. If some ofthe variations in contact adhesion to vary with time, the actual sampledresolution will be lower than theoretical. Contact conditioning is anexample of variation that may change with time. Contact aging could alsocontribute to this variation.

In the first operation embodiment, the array of switches would be set tothe closed state at the start of the sample period and then read againat the end of the sample period. By performing the sampling at a ratefaster than the highest frequency of interest in the analog inertialinput signal, an accurate representation of the input can be achieved(sample & hold technique) as shown in FIG. 9 where the measuredacceleration at different time intervals is shown.

The output of the switch array is always a digital signal by its verynature so an automatic analog to digital conversion is achieved. Anexample of this conversion is shown in Table I:

TABLE I Time Step Percentage of Percentage of Digital Difference (DT)switches open switches closed (10,000 switches) 1 50% 50% 0 2 60% 40%2000 3 66% 34% 3200 4 49% 51% −200 5 38% 62% −2400

Calibration of the digital response during final test may be required asproduced however; self calibration in the field is also easily performedduring power-up. Biasing a section of the array to have more asymmetrythrough changes in the torsion legs, contact area, or any of the otherdesign parameters can be used to artificially widen the differencedistribution if a greater range of accelerations are needed. Increasingthe number of switches can be used to produce a great sensitivity. Thisbasic technique of using a switch array to subsample a differencedistribution of adhesion can be extended to measure analog voltages asthe input signal by sampling the voltage on the pull-in electrode.

An alternative embodiment of the present invention to implement thearray accelerometer would have the same design as in the previousembodiment, but each switch would be held in the off state. Theadvantage of this embodiment is that the distribution in adhesion energyis no longer present and so the distribution can be made narrower. Then,an additional electrostatic force (FE), caused by a pull in voltage, isapplied to the tuning electrode to bring the cantilever close toswitching. At this point the cantilever will switch when the chip issubject to a small acceleration. Again because of the distribution inthe array turn on voltage, the number of switches switching will be ameasure of the level of acceleration. The switch becomes closed when:FE*a−M*A*a>Ft*b

In this case if the acceleration is negative (i.e., up), then thecantilever will switch. The switch remains open when:FE*a−M*A*a<Ft*b

With no voltage is applied, the cantilever sits in an energy well whichis parabolic in nature. The torsion restoring force is linearlydependent on displacement angles for small displacements. Anacceleration must be large enough to move the torsion cantilever untilthe contact 410 is made to a landing electrode 404 on the right handside of FIG. 4. When this contact is made the adhesion forces keep thecantilever in the on state, where the resistance is measured and thecantilever is said to have switched. This cantilever can then beelectrostatically reset with a voltage applied to an electrode under theleft hand arm (electrode 405 FIG. 4). The cantilever is now reset andcan measure the next acceleration force.

Because these small cantilevers have a small mass the electrostaticswitching time can be very quick allowing the cantilever to be reset in200 ns. This off time is very small in comparison to most mechanicalvibration frequencies that may need to be measured. By adding anadditional electrostatic force by grounding the cantilever and applyinga pull up voltage on electrode 401 shown in FIG. 4, the cantilever isrotated so that the cantilever has less distance to be acceleratedacross before it sticks to the landing electrode. In this way, thesensitivity of the cantilever can be electrically controlled. With aconstant voltage difference between the pull up electrode and thecantilever proof mass, the attractive force increases as the cantileverproof mass gets closer to the pull up electrode because the electricfield increases. The restoring force from the torsion arm is linearlydependent on this distance, so this pull-up voltage effectively alsoreduces the restoring force. This makes the movement of the proof massgreater for the same external acceleration which also increases thesensitivity of the individual accelerometer, or alternatively we can saythe pull up voltage on electrode 401 reduces the spring constant of thecantilever.

A simple model of the force on a cantilever of length L and width w witha gap d0 that deflects by a distance x and has a spring constant of thecantilever k is given by:

$F_{Total} = {{m\;{A(t)}} + {\frac{ɛ\;{Lw}}{2\left( {{d\; 0} - x} \right)^{2}}V^{2}} - {kx}}$

where V is the applied voltage on electrode 401. This produces aneffective spring constant:

$k_{Eff} = {k - {\frac{ɛ\;{Lw}}{\left( {{d\; 0} - x} \right)3}V^{2}}}$

Increasing the magnitude of the voltage on electrode 401 will reduce theeffective spring constant of the cantilever making it more sensitive toacceleration as well as moving the cantilever closer to the landingelectrode, so that less movement is required before switching.

The advantage of operating in this mode is that time or switchingdependent change in the adhesion force does not cause a change in thesensitivity. The device can be operated in two manners.

In a first manner, a pull in voltage is applied so half the cantileverspull in, and then the acceleration causes extra cantilevers to turn on.The number switching is then recorded and all the cantilevers reset andpulled in so that half the cantilevers are on. The cantilevers are thenpulled off and half pulled in. This process is repeated continuously inthe same sample and hold technique as above. Initially the voltagerequired to switch half the cantilevers on with no acceleration appliedis ascertained during the manufacturing process and this voltage isstored on chip.

In a second manner, all the cantilevers are pulled in and the thresholdof each is logged. Then all the cantilevers are pulled in again and thethreshold shift of each is measured again. The total shift in thresholdvoltage is then a measure of the acceleration and this can be measuredwith a precision that is √{square root over (N)} better than theprecision that one cantilever can be measured.

The above techniques require the cantilevers to switch at a very highrate, though the sensitivity of measurement is very high because thesensing uses the change of resistance between the on state which can be1 to 10 K Ohms and the resistance of the off state which will be greaterthan 10 M Ohms. Such a change in resistance can be measured in 100 nsquite easily. This means that the state of the cantilever can bemeasured and reset in less than 200 ns giving a band width of more than1 M Hz. Given that most mechanical accelerations that need to bemeasured are 10 K Hz or less this allows the acceleration to be measuredover 100 times to give and additional sensitivity improvement of atleast 10.

Another embodiment of the present invention of using an array of smallercantilevers to measure acceleration instead of one larger cantilever isto measure the change in capacitance between all the proof masses andall the electrodes like electrode 1 under them. This is adding Ncapacitances in parallel which makes it N times bigger. The fundamentallimit on measurement is given by thermal vibrations of the cantilever.This is true for a large accelerometer and it would also be the case forN small cantilevers acting in parallel. The movement of the cantileveris sensed using the change in capacitance between the proof mass and afixed electrode. As the capacitance is linearly dependent on the changein gap then the movement and thus acceleration can be deduced from thechange in capacitance. A differential capacitance sensing circuit asused for single large cantilevers could equally be used for N smallcantilevers measured as capacitors in parallel.

One of the fundamental limits on the sensitivity of an accelerometer isgiven by the thermal activation of vibrational modes. The mean squareforce noise is given by √{square root over (4k_(B)TB)} where B is thedamping term for a spring dashpot model. This can be described in termsof the resonant frequency ω₀ (which is equal to K/m where K is thespring constant and m the mass of the cantilever) and Q the qualityfactor (which is described by

$\left. \frac{m\;\omega_{0}}{B} \right).$The displacement is proportional to the acceleration and inverselyproportional to the square of the resonant frequency.

$x = \frac{a}{\omega_{0}^{2}}$Thus the Brownian motion noise will lead to acceleration noise given by:

$a_{RMS} = \sqrt{\frac{4k_{B}T\;\omega_{0}}{m\; Q}}$

For a single large cantilever with a 20 kHz resonant frequency and amass of 2×10⁻¹⁰ kg and a Q of 5 set by squeeze film damping, thesensitivity will be 5×10⁻³ m/(s² √{square root over (Hz)}).

For a simple cantilever of length L, width w and thickness t we canrewrite this as:

$\omega_{0} = {{\frac{3.52}{L^{2}}\sqrt{\frac{EI}{\left( {m/L} \right)}}} \approx \sqrt{\frac{{Et}^{2}}{\rho\; L^{4}}}}$

Inserting this equation into the a_(RMS) equation, we get for a_(RMS):

$a_{RMS} = {\sqrt{\frac{4k_{B}T\;\omega_{0}}{m\; Q}} = {\sqrt{\frac{4k_{B}T}{\; Q}} \cdot \left( \frac{E}{\rho^{3}w^{2}L^{6}} \right)^{1/4}}}$

For a plate of width w₁ and length b, the mass moment of inertia i aboutone end is given by (⅙)mb². We can approximate the mass moment ofinertia of the proof mass about the rotational axis that runs throughthe torsion arms as (⅙)mb².

The spring constant of one of the two torsion springs, which consist ofa beam of length L width w₂ and thickness t₂ is:

$k = \frac{{Gw}_{2}{t_{2}\left( {w_{2} + t_{2}} \right)}}{12L}$

where G is the shear modulus (119 G Pa for TiN). The resulting resonantrotation frequency about the center of rotation is:

$f = {\frac{1}{2\pi}\sqrt{\frac{2k}{i}}}$

where there are two torsion arms, one on each side.

With regard to the top view of the device defined below we can use thefollowing dimensions: L=4 microns, w₂=0.5 microns, and t₂=30 nm. For theproof mass, the thickness t=0.5 microns, the width w=4 microns, andlength of the proof mass b=4 microns. For these values the resonantfrequency is 1.6 M Hz. The mass of the proof mass would be 4.4×10⁻¹⁴ kgusing

$a_{RMS} = \sqrt{\frac{4k_{B}T\;\omega_{0}}{m\; Q}}$

This gives a thermal noise for the acceleration of 0.87 ms⁻²/√{squareroot over (Hz)} for a Q of 5. Changing these values only slightly toL=4.5 microns, w₂=0.35 microns, and t₂=30 nm.

For the proof mass, the thickness t=0.5 microns, the width w=4 microns,and length of the proof mass b=5 microns, then we get a resonantfrequency of 618 KHz with a proof mass of 6×10⁻¹⁴ kg. This gives athermal noise for the acceleration of 0.5 ms⁻²/√{square root over (Hz)}.

FIG. 11 is a top view of the torsion cantilever with the two torsionside arms 1103 and the large proof mass 1102 to the left shown. Thelanding contact 1110 is to the right. This is for one cantilever. For Ncantilevers this would improve by √{square root over (N)}. The earliersingle large accelerometer example is for an accelerometer of the orderof 450 microns by 450 microns. If we can fit the device in an area of 10by 8 then we fit 2,000 devices in such an area. This would mean thenoise levels would be a factor of 2 larger in our array when no biasvoltage is applied.

Finally we could apply a pull in voltage to put us close to theswitching point. This makes the potential minima the cantilever sits inmore shallow (as discussed above), this further reduces the resonantfrequency and reduces the acceleration noise allowing us to achieve thesame sensitivity.

Thus, by design and operation, we reduce the resonant frequency to 200KHz which would make the acceleration noise of each cantilever 0.1 m/(s²√{square root over (Hz)}) or 10 mg/√{square root over (Hz)} percantilever. Using

${x = \frac{a}{\omega_{0}^{2}}},$that gives a maximum noise displacement of 1 nm. Given that we have a 30nm gap this gives us a maximum acceleration that we can handle of 30 gwhen the device is not powered up.

It is useful to note that if we scale thickness, length, and width of acantilever by β, then we can replace one large cantilever with β²smaller cantilevers then acceleration noise or minimum sensitivity ofeach cantilever will go up as

$a_{RMS} \propto \frac{1}{\beta^{2}}$

However the number of cantilevers goes up as β² so the accelerationnoise can be averaged out by a factor β. Thus scaling one largecantilever into N smaller ones leads to an increase in averageacceleration noise of:

$a_{RMS} \propto \frac{1}{\beta}$

This dependence on scaling of the sensitivity can be offset partially asthe quality factor is expected to improve with the loss of squeeze filmdamping for smaller gaps and by softening of the mode with an appliedvoltage and by a change of design to a torsion sensor.

Because of the scaling of the gap under the cantilever the same electricfields can be generated using lower voltages, which make it easier toelectrically integrate with the native CMOS voltages.

A typical proof mass cantilever accelerometer coupled to interlockingfinger capacitance sensing, would have a capacitance given by:

$C \approx {100\; f\;{F\left\lbrack {1 \pm \frac{y}{G_{0}}} \right\rbrack}}$

where y is the displacement and G₀ is the gap between fingers (about 1micron). An estimate for the capacitance of a torsion device can bedetermined by setting C=∈wL/G and estimating the gap to change linearalong the cantilever G=d₀+y*x/L. Integrating along the length, we getthe total cantilever capacitance as a function of tip displacement y as:

$C = {{\frac{ɛ_{0}w\; L}{d_{0}} \cdot \frac{\ln\left( {1 + {y/d_{0}}} \right)}{y/d_{0}}} \approx {\frac{ɛ_{0}w\; L}{d_{0}}\left( {1 - \frac{y}{2d_{0}}} \right)}}$

For the scaled cantilever array devices envisaged here we would have

$2000\frac{w\; L\; ɛ_{0}}{d\; 0}$where w is the width of the cantilevers, L the length and d0 the gap.With w=5 microns L=5 microns and d=300 nm, we get a total capacitanceof:

$C \approx {1300\; f\;{F\left\lbrack {1 \pm \frac{y}{600\mspace{14mu}{nm}}} \right\rbrack}}$

This is a factor of 5 larger than a standard accelerometer. Thus foreach cantilever we have:

$C \approx {0.65\; f\;{F\left\lbrack {1 \pm \frac{y}{300\mspace{14mu}{nm}}} \right\rbrack}}$

There is thermal noise leading to 1 nm displacements, and there willalso be thermally generated electrical noise that may double this, sofor an individual cantilever the minimum detectable capacitance would be( 1/30) 0.6 fF. Or with N=2,000 cantilevers that would be 1 aF.Estimates for a single large cantilever with finger capacitancedetection gives a value of 1 aF/nm for the capacitance noise, whichmatches the multi accelerometer array.

The capacitance would be measured using a differential technique. Forvertical displacement the capacitance between the proof mass 402 in FIG.4 and the top electrode 401 is one capacitor and the other is thecapacitance between the proof mass and the electrode 405 at the bottom.By ensuring theses two capacitances have the same value by designing theareas and gaps appropriately, then we measure the small differencebetween these two in a bridge circuit.

In FIG. 12, C1 is the capacitance between the proof mass 402 and the topelectrode 401, while C2 is the capacitance between the proof mass andthe bottom electrode 405. With a positive voltage on 405 and a negativevoltage on 401, the output Vo is zero on balance. The output is thenjust proportional to the change in the position of the proof mass in thecavity.

Looking at noise contributions to arrays of cantilevers, it would appearthat there is not a fundamental physics limit to making an array ofcantilevers as sensitive an accelerometer as a single micro machineddevice, of the same area. Because the present invention allows embeddingof MEMS devices, such as an accelerometer on an existing CMOS chip,there is a huge saving in space associated with all the periphery ofbond pads and electronics which would make the device 10 times bigger asa stand alone chip.

We can make this device into a 3D accelerometer by using two layermetallization and hang vertical proof mass at one end of a torsionspring. This shifts the center of mass above the fulcrum allowinglateral accelerations to generate vertical movement at one end of thecantilever. With ⅓ of the array with the rotation axis at right anglesto the other ⅓ of the cantilevers. The final ⅓ would be flat with theproof mass in plane. Then an acceleration in the x direction will onlyrotate those cantilevers with the rotation axis along the y axis thathave a proof mass out of plane. The cantilevers with the rotation axisalong the x axis with the proof mass out of plane will not be affectedby acceleration along the x axis. With the cantilever with no mass outof plane will not be affected by lateral accelerations.

FIG. 13 is an example of a small accelerometer from an array, which issensitive to acceleration in the x-y plane parallel to the substrate.The accelerometer includes a pull up electrode 1301, the pull downelectrode 1305, the proof mass 1302, the torsion arm 1303, the landingpoint 1310, the interlayer via for a standard back end metallization1314, the interlayer dielectric 1315, and the metallization layer for astandard back end CMOS process 1313, the next interlayer dielectric1316.

Advantages of the present invention include the capability to have fullyintegrated packaging into the BEOL of a semiconductor process as opposedto a separate package that is required now and noise immunity from thesmall size of each switch. Each switch in the array has a higherresonant frequency compared to a single large MEMS device, and so isless prone to vibration noise at low frequencies. Another advantage isintegrated analog to digital conversion since the sampled switch count(digital) represents the magnitude of the input signal (analog). Anotheradvantage is decreased cost over competing solutions because it can beembedded in the back end and does not need a separate package. Anotheradvantage includes the device can be integrated with standard ICsbecause it can be encapsulated in the back end processing, which is notpossible with the existing larger single MEMS devices.

By making the MEMS switches that form the capacitors smaller, therestoring forces per unit area can be made larger, because the restoringforces of a cantilever becomes larger as the cantilever is made shorter.The increased restoring force per unit area at the contact, helpsprevent unwanted switching caused by high RF signals applied to thecapacitor signal line. The AC voltages lead to an average attractiveforce that can be quite high. Larger MEMS switched capacitors can bepulled in more easily. It is also important to be able to pull thecapacitor off the landing electrode when RF power is applied. Theincreased pull off force per unit area in the smaller MEMS switch helpswith this as well.

Additionally, the device life may be extended by having a plurality ofsmaller switches rather than a single, large switch. Specifically, whenthere are a plurality of switches, the device may still function whenone or more of the smaller switches ceases to function so long as atleast one switch continues to function. However, when a single, largeswitch is used, the device is inoperable once the switch ceases tofunction. Thus, when a plurality of smaller switches are utilized, notonly can the device performance be improved, but the device lifetime maybe lengthened.

To solve the problem with being able to switch off the MEMS capacitorwhen RF power is applied larger MEMS RF capacitor devices need tointroduce a voltage divider which complicates the design and increasesthe cost of production. By making the capacitors smaller with a greaterpull off force per unit area this problem is solved.

Because each MEMS capacitor is smaller, it has a lower mass and can beswitched more quickly. This gives a faster response time and quickerability to tune the resulting RF signals. With a normal MEMS variablecapacitor, the capacitance can be controlled by controlling the gapbetween a cantilever and the RF signal line. The resulting capacitor hasnoise associated with mechanical vibrations of the cantilever which isnot present with a digital variable capacitor when it is fully switchedon as the MEMS cantilever is then in contact with the dielectric layerover the RF signal line.

By breaking the capacitor up into an array of smaller capacitors, theyield can be improved. This is because if one MEMS device of the arrayof digital capacitors is not switching, the array will still provide avariable capacitor with a slight shift in the specifications. With onelarger device, a problem with a point like defect can prevent the wholedevice from working, resulting in no capacitance change at all.

By making the cantilevers smaller, one can interconnect the devicesinside the cavity which ensures that the stray capacitance andelectromagnetic fields can be controlled. Because the capacitor has beenbroken up into an array of smaller cantilevers, there is a great deal ofdesign freedom in the shape of the array. It can be set out in a longnarrow array in-between the RF and Ground lines which helps reduceunwanted coupling from stray electromagnetic fields (i.e., the array canbe fitted in a strip line arrangement). This would be very hard with asingle capacitor as you do not have so much freedom in the aspect ratioof the design of a single MEMS cantilever capacitor.

FIG. 14 is a top view of the control electrodes 1402 and the RFelectrode 1404. FIG. 15 is a top view of the cantilever 1506 with foursupport arms 1502 over the top of the RF line 1504. The cantilever 1506provides a path to ground and is shown over the top of the RF line 1504and control electrodes. The control electrodes function at first andsecond electrodes while the RF line 1504 functions as a third electrode.The cantilever 1506 pivots or moves from a position spaced a firstdistance from the RF line 1504 to a second position spaced a seconddistance from the RF line 1504. The first distance is greater than thesecond distance. The distance between the RF line 1504 and the groundedcantilever 1506 creates a capacitance. The smaller the distance betweenthe RF line 1504 and the grounded cantilever 1506, the greater thecapacitance.

FIG. 16 is a top view of multiple MEMS capacitors arranged along an RFelectrode, labeled “RF” which can be included inside a single cavity.Two control electrodes, labeled “CNT” are also present. The individualMEMS capacitors utilize a common RF electrode and common controlelectrodes. The support arms are coupled to ground, which is labeled“GND”. The RF-GND arrangement implements a coplanar structure whichconfines electromagnetic fields inside the cavity minimizing strayeffects. As shown in FIG. 16, there is a plurality of cantilevers thatare each adjacent to the common RF line. Additionally, there are twocontrol electrodes, which may be referred to as first and secondelectrodes. The plurality of cantilevers pivot or move between aposition spaced a first distance from the RF line and a position spaceda second distance from the RF line. The distance that the groundedcantilevers are spaced from the RF line determines the capacitance.

As shown in FIG. 16, there is a first electrode, a second electrode anda RF line that is common for the entire structure. The plurality ofcantilevers share the common first, second and RF electrodes.Additionally, the cantilevers and electrodes collectively function as adigital variable capacitor and may be contained within a single cavitywithin a device structure. For example, the digital variable capacitormay be embedded within a cavity within a CMOS device such that anelectrical connection is made from below the digital variable capacitorto a location above the cavity and the digital variable capacitor.

FIGS. 17A-17C are schematic cross sectional views of a MEMS capacitorswitch 1700 in the free standing position, the down state and the upstate according to one embodiment. In FIGS. 17A-17C, the switch 1700 hasmultiple ground electrodes 1702, a first pull-down electrode 1704, an RFelectrode 1706, a second pull-down electrode 1708, an insulating layer1716 formed over the pull-down electrodes 1704, 1708 and the RFelectrode 1706, a pull-up electrode connection 1710, an insulating layer1712 disposed over the pull-up electrode 1718, and a cantilever 1714. Inone embodiment, the insulating layers 1712, 1716 may comprise an oxide.In FIG. 17A, the cantilever is in the free standing position spaced fromthe oxide layer 1716 over the electrodes 1704, 1706, 1708 and spacedfrom the oxide layer 1712 formed over the pull-up electrode 1718.

In FIG. 17B, the cantilever 1714 is in the down state where thecantilever 1714 is in contact with the insulating layer 1716 thatoverlies the electrodes 1704, 1706 and 1708. The cantilever 1714 isspaced a small distance from the RF electrode 1706 by the insulatinglayer 1716 and thus, a large capacitance is present because thecantilever 1714 is grounded and the RF electrode 1706 is RF hot. Whenthe cantilever 1714 is in the up state and in contact with theinsulating layer 1712 formed over the pull-up electrode 1718, a largedistance is present between the RF electrode 1706 and the cantilever1714 which is grounded. Thus, there is a small capacitance in FIG. 17Crelative to FIG. 17B. The switch 1700 becomes a digital variablecapacitor with each smaller capacitor having two states. The first stateis where the cantilever 1714 is pulled down touching the thin insulatinglayer 1716 over the RF electrode 1706. The second state is where thecantilever 1714 is pulled up in contact with the oxide layer thatoverlies the pull-up electrode 1718. The capacitance is given by:C=A∈∈ _(o) /d

where A is the area of overlap between the RF line and the cantilever, ∈is the dielectric constant, ∈_(o) is the relative dielectric constant ofthe material between the RF line and the cantilever and d is thedistance between the cantilever and the RF line. In one embodiment, d isbetween about 0.2 microns and about 1.0 microns. In another embodiment,d is between about 100 nm to about 1 micron. In one embodiment, thecavity in which the device is disposed may have at least one dimension(i.e., length, width or height) that is between about 20 microns toabout 30 microns. In another embodiment, the cavity may have at leastone dimension that is up to about 300 microns. In one embodiment, thecavity may have at least one dimension that is between about 140 micronsand about 155 microns. In cavities that have at least one dimension thatis greater than about 30 microns, support posts may be present in thecavity to support the roof of the cavity. In some embodiments, supportposts may not be present.

In order to function as a capacitor, the pull down electrodes 1704, 1708may provide a first voltage. In one embodiment, the pull down electrodes1704, 1708 provide the same voltage. Simultaneously, the pull-upelectrode 1718 may provide a second voltage in opposition to the firstvoltage. Thus, the cantilever 1714 may be pulled down into contact withthe insulating layer 1716 because the cantilever 1714 is pulled in bythe pull down electrodes 1704, 1708 and repelled by the pull-upelectrode 1718. Similarly, the voltages may be reversed to move thecantilever 1714 to the position in contact with the insulating layer1712.

When operating as a digital variable capacitor, such as shown in FIG.16, a plurality of cantilevers are coupled to ground and disposed abovean RF electrode and one or more pull-down electrodes. One or morepull-up electrodes are also present. The cantilevers move between twostates. The first state has the cantilever spaced a small distance fromthe RF electrode. The second state has the cantilever spaced a greaterdistance from the RF electrode. The grounded cantilever is never indirect contact with the RF electrode. It is the spacing between the RFelectrode and the grounded capacitor that creates a capacitance.

The advantage of having a large number of smaller capacitors rather thana few larger ones is that the capacitors can be made with smallerdimensions so that they can be housed in a cavity in the back endmetallization layers of a normal CMOS process. If they are made toolarge the cavity then becomes too large and the residual stresses in thecavity roof makes them buckle up or down. This problem could be solvedwith layers thicker than one micron, but then it becomes increasinglydifficult to process the cavity in a standard back end process. Thisthen increases costs.

While the foregoing is directed to embodiments of the present invention,other and further embodiments of the invention may be devised withoutdeparting from the basic scope thereof, and the scope thereof isdetermined by the claims that follow.

The invention claimed is:
 1. A device structure, comprising: asubstrate; a plurality of layers formed over the substrate, a firstlayer of the plurality of layers bounding one or more cavities formedwithin the structure between the substrate and the plurality of layers;a plurality of micro electromechanical devices disposed over thesubstrate and within each of the one or more cavities; a pull upelectrode within at least one of the one or more cavities and coupledwith the first layer above the plurality of micro electromechanicaldevices; and a via connection from the substrate to one or more layersdisposed over the one or more cavities; wherein the one or more cavitiesand plurality of micro electromechanical devices are embedded within acomplementary metal oxide semiconductor; and wherein a first device ofthe plurality of devices has a different design than a second device ofthe plurality of devices.
 2. The device structure of claim 1, wherein atleast one of the one or more cavities has a length, a width and a heightwhere at least one of the length or width is between about 20 micronsand about 30 microns.
 3. A device structure, comprising: a substrate; aplurality of layers formed over the substrate, a first layer of theplurality of layers bounding one or more cavities formed within thestructure between the substrate and the plurality of layers; a pluralityof micro electromechanical devices disposed over the substrate andwithin each of the one or more cavities; a pull up electrode within atleast one of the one or more cavities and coupled with the first layerabove the plurality of micro electromechanical devices; a via connectionfrom the substrate to one or more layers disposed over the one or morecavities; a first electrode; a second electrode; and a plurality ofcantilevers that are each movable from a position in contact with thefirst electrode to a position spaced from both the first electrode andthe second electrode to a position in contact with the second electrode.4. The device structure of claim 3, wherein the device structure is adigital variable capacitor.
 5. The device structure of claim 4, furthercomprising a third electrode, wherein each of the plurality ofcantilevers is coupled to the third electrode.
 6. A device structure,comprising: a substrate; a layer formed over the substrate and boundinga cavity formed within the structure; a plurality of microelectromechanical devices disposed within the cavity; a pull upelectrode within the cavity and coupled with the layer; a via connectionfrom the substrate to layer; a first electrode; a second electrode; anda plurality of cantilevers that are each movable from a position incontact with the first electrode to a position spaced from both thefirst electrode and the second electrode to a position in contact withthe second electrode, wherein: the cavity has a length, a width and aheight where at least one of the length or width is between about 20microns and about 30 microns; the cavity and plurality of microelectromechanical devices are embedded within a complementary metaloxide semiconductor; and a first device of the plurality of devices hasa different design than a second device of the plurality of devices. 7.The device structure of claim 6, wherein the device structure is adigital variable capacitor.
 8. The device structure of claim 7, furthercomprising a third electrode, wherein each of the plurality ofcantilevers is coupled to the third electrode.
 9. A device structure,comprising: a substrate; a layer formed over the substrate and boundinga cavity formed within the structure; a plurality of microelectromechanical devices disposed within the cavity; a pull upelectrode within the cavity and coupled with the layer; a via connectionfrom the substrate to layer; a first electrode; a second electrode; anda plurality of cantilevers that are each movable from a position incontact with the first electrode to a position spaced from both thefirst electrode and the second electrode to a position in contact withthe second electrode.
 10. The device structure of claim 9, wherein thedevice structure is a digital variable capacitor.
 11. The devicestructure of claim 9, further comprising a third electrode, wherein eachof the plurality of cantilevers is coupled to the third electrode. 12.The device structure of claim 10, further comprising a third electrode,wherein each of the plurality of cantilevers is coupled to the thirdelectrode.